There are various methods for detecting defects in, and thus testing the future reliability of, thin film insulators in integrated circuits, particularly insulators in memory devices. Memory devices include EPROMs, EEPROMs, DRAMs, and other products with nonvolatile memory. Unfortunately, existing wafer-level-reliability monitors of oxide breakdown voltage are not good predictors of oxide reliability, are slow, and are destructive.
One type of reliability testing is called "burn-in". In a burn-in test, the integrated circuit is subjected to elevated temperatures before performing functional tests on the integrated circuit. Currently, an integrated circuit undergoing a burn-in test is subjected to an elevated temperature for several hours. Because a large amount of time is required for a burn-in test, functional testing is usually performed before a burn-in test is performed. Later, burn-in tests and other tests for material defects are performed on samples in a group, or batch, of integrated circuits which have already undergone functional testing.
Scanning electron microscope ("SEM") views of semiconductor surfaces provide detailed information about the structure of the device, but require the destructive slicing of the die to expose the cross sectional view. Much time is required to prepare and analyze the many SEM photographs needed to adequately represent an entire semiconductor.
Charge-to-breakdown ("QBD") measurements force a constant current through a MOS capacitor and monitor the voltage across the capacitor until the insulator saturates, anywhere from five to thirty minutes later. This test destroys the capacitor, and is not sensitive to any bursts of current. These QBD measurements also do not correlate well with oxide failures.
Gate-oxide-integrity (GOI) tests apply ever-increasing "steps" of voltage across a MOS capacitor and monitor the current at each step. This test takes about 30 seconds, and although it gives an indication of the average strength of the oxide, it suffers the limitation that it destroys the capacitor, and it stresses the oxide with much higher electric fields than those applied in a typical application, thereby not testing for defects which cause reliability failures at low electric fields. The GOI test result also depends on step duration as well as voltage.
Other methods of oxide characterization rely on DC measurements which typically only detect the minimum Si--SiO.sub.2 barrier height, and in fact may miss the true minimum if self-healing occurs. A short test method is needed to predict oxide properties in order to improve reliability.
Sze, in Chapter 7 of his text, "Physics of Semiconductor Devices", 2d Ed., summarizes basic conduction processes in insulators, listing (in table 4 on page 403) six conduction processes, three of which depend on a factor called "barrier height". Interface currents dominated by barrier height have been shown by Guttler and Werner to be related to the noise power density. A similar effect is observed for thin insulators which conduct by Fowler-Nordheim tunneling.
Several studies have found that the noise level is governed by the interface-state density and in general suggested the noise in the MOS transistor is a surface effect.
Amberiadis et al. conducted noise measurements in P.sup.-- type diffused and ion-implanted MOS capacitors. They found that there are three sources of 1/f noise: (1) number fluctuation 1/f noise (due to the interaction of holes with the surface oxide); (2) bulk mobility fluctuation 1/f noise (due to the fluctuation in the bulk mobility); and (3) modulation 1/f noise (when inversion becomes important electrons in the inversion layer begin to interact with oxide traps, giving rise to fluctuations in the surface potential, which in turn gives a 1/f modulation of the surface mobility, or a direct modulation of the bulk resistance). Kostas Amberiadis and Aldert Van Der Ziel, "1/f Noise in Diffused and Ion-Implanted MOS Capacitors", Solid-State Electronics, Vol. 26, No. 10, pp. 1009-1017 (1983).
Neri et al. disclose a method for measuring low-frequency noise, and state that "low-frequency noise measurement is a very useful, non-destructive, and alternative tool in analyzing electrical and structural properties of electron devices and materials", and that weak spots in thin thermal oxide "are characterized by a notable local lowering of the barrier height". B. Neri, P. Olivo, and B. Ricco, "Low-frequency noise in silicon-gate metal-oxide-silicon capacitors before oxide breakdown", Appl. Phys. Lett., Vol. 51, No. 25, pp. 2167-2169 (1987). However, they do not describe how the measurement determines barrier height, or how it determines the minimum barrier height, or how those measurements affect the reliability of the electron device.
Butler et al. have derived an expression for the drain voltage noise for an N.sup.-- channel MOSFET using a trap-induced-number-fluctuation theory. Z. Celik-Butler and T. Y. Hsiang, "Spectral Dependence of 1/f.gamma. Noise On Gate Bias In N.sup.-- MOSFETS", Solid-State Electronics, Vol 30, No. 4, pp. 419-423 (1987). However, they do not discuss any such relationship for capacitors.
Van Der Ziel states that "in MOSFETs electrons tunnel from traps in the oxide, at a distance z from the interface, to the conducting channel and vice versa. . . . The electrons must climb a potential barrier before they can reach the surface and interact with oxide traps. The 1/f noise is therefore reduced, and this becomes more pronounced near saturation. . . . Most MOSFETs have surface 1/f noise and most bulk semiconductor resistors have volume 1/f noise. . . . Surface 1/f noise can be turned off and on. The trapping of carriers in the surface oxide can also give rise to surface potential fluctuations, and hence to mobility 1/f noise." Aldert Van Der Ziel, "Unified Presentation of 1/f Noise in Electronic Devices: Fundamental 1/f Noise Sources", Proceedings of the IEEE, Vol. 76, No. 3, pp. 233-258 (1988).
Young et al. have disclosed that "nitride-oxide stacked films can be thought of as an oxide film with electron trapping at the nitride/oxide interface. The density of electron trapping is determined by the current-continuity requirement." K. K. Young, Chenming Hu, and William G. Oldham, "Charge Transport and Trapping Characteristics in Thin Nitride-Oxide Stacked Films", IEEE Electron Device Letters, Vol. 9, No. 11, pp. 616-618 (1988).
Wong & Cheng state that "it is possible to apply the low-frequency noise measurement to estimate the oxide-trap distribution. . . . Low-frequency noise in MOS systems indeed originates from the oxide traps, at least partially if not dominantly, rather than solely from interface-state density." H. Wong and Y. C. Cheng, "Gate dielectric-dependent flicker noise in metal-oxide-semiconductor transistors", J. Appl. Phys., Vol. 67, No. 2, pp. 863-867 (1990).
Electronic properties of Schottky diodes depend sensitively on spatial inhomogeneities of the metal/semiconductor interface. Guttler et al. disclose that "excess noise increased drastically when the standard deviation .sigma..sub.s of the spatial distribution of Schottky barrier heights exceeds the critical threshold value of 2 kT". Herbert H. Guttler and Jurgen H. Werner, "Influence of Barrier Inhomogeneities on Noise at Schottky Contacts", Appl. Phys. Lett., Vol. 56, No. 12, pp. 1113-1115 (1990).
Dreyer et al. disclose a method for measuring the integrity of semiconductor multi-layer metal structures. They disclose "forcing direct current flow through a semiconductor metal structure. Voltages with generally non-periodic frequencies are generated by the current flow and are converted to a voltage spectral density. This voltage spectral density is then compared to the voltage spectral density of a defect free metal structure. A larger voltage spectral density value for the test sample indicates the presence of defects in its metal structure." U.S. Pat. No. 5,049,811, entitled "Measuring Integrity of Semiconductor Multi-layer Metal Structures", to Dreyer et al. (1991).
Gutt et al. also disclose testing metal films by measuring 1/f noise. They disclose "causing a direct current and an alternating current to flow in the test portion, the combined currents being of sufficient magnitude to stimulate 1/f.sup.2 noise in the test portion; determining the noise spectrum associated with the alternating current; and comparing the slope and amplitude of the spectrum with predetermined values at one or more preselected frequencies." U.S. Pat. No. 5,057,441, entitled "Method For Reliability Testing Integrated Circuit Metal Films", to Gutt et al. (1991).
Dreyer et al. and Gutt et al. disclose testing a conductor by passing current through it. Thus, this is somewhat duplicative of what occurs in the normal operation of that conductor.
Currently functional tests are performed on integrated circuits before those integrated circuits are tested for material defects. This is because the testing for material defects takes too much time. The unfortunate result of this compaction of production scheduling is that during or after functional testing of an integrated circuit, it will be discovered that the integrated circuit has a material defect, such as a defective MOS capacitor. Each functional testing machine costs in excess of $2 million. Thus, because of the usually limited number of functional testers, functional testing becomes a bottle-neck in the production of integrated circuits. If integrated circuits having material defects could be discovered before functional testing, this would speed up production of integrated circuits, because functional testing would not be performed on defective integrated circuits. Thus, a need exists in the art for a way to quickly test the material of an integrated circuit before that integrated circuit undergoes functional testing.
The invention overcomes the above-noted and other drawbacks of the prior art by providing a method and apparatus for determining the reliability of insulators in integrated circuits with noise measurements. The preferred embodiment of the method of the present invention uses all current flow as it contributes to the complete noise spectrum. Noise measurements of oxide current can give the mean and sigma of the barrier height which regulates tunneling. This is very sensitive to oxide defects which allow some current to flow, then self-heal, because the noise spectrum is broadened by temporal spikes. These current spikes can cause failures in floating gate and DRAM memory cells.
The preferred embodiment of the method of the present invention provides a true reliability indicator which can be used for sample screening and process improvement. The preferred embodiment of the method of the present invention eliminates the undesirable feature common to existing methods wherein the MOS capacitor being tested is destroyed. The invention is the first method for testing the reliability of semiconductor insulators which does not have to wait until the device fails before a reliability determination can be made. The present invention is believed to be the first method that allows for testing of the material of every integrated circuit being produced before that integrated circuit undergoes functional testing. The method of the present invention is further believed to be the first method which allows for testing the material of the integrated circuit during the actual production of the integrated circuit, thus allowing improvements to be made in the production of an integrated circuit while that particular integrated circuit is being produced.